When the SelectionMode of RadAutoCompleteBox is changed, the search text is cleared. However, this clears only the Text property of the underlying TextBox element. The SearchText property of the control is not cleared.

this.autoCompleteBox.SelectionMode = AutoCompleteSelectionMode.Multiple;
this.autoCompleteBox.SearchText = null;

By default the TextSearchPath of RadAutoCompleteBox is automatically assigned when the DisplayMemberPath is set, and if the TextSearchPath is not yet assigned. However, this happens only initially. Runtime changes of the DisplayMemberPath don't update the text search path.

To work this around, set the TextSearchPath manually when you change the DisplayMemberPath.